FIELD OF THE INVENTION
The present invention relates to a method and apparatus for the monitoring and control of the concentration of ions, especially hydrogen ions, in process streams at elevated temperatures. In a particular embodiment, the method and apparatus relate to monitoring and control of pH in processes for the manufacture of polyamides.
Polyamides are produced by thermal condensation of diacids with diamines. The ratio of the diacid and diamine monomers must be very nearly stoichiometric in order to obtain a polymer having a high molecular weight. In practice, a concentrated solution of the salt of the diacid and diamine in water is prepared. The pH of this solution is very sensitive to the ratio of the two monomers and pH measurements are used to adjust the ratio with a high degree of accuracy. This may readily be accomplished in the case of the lower polyamides, e.g. polyhexamethylene adipamide which is also known as nylon 6/6, because 1,6-diaminohexane adipate salt is very soluble in water at ambient temperature. Concentrations of the salt of 50%, by weight, may easily be achieved, and are used as the feed stock for the polymerization process. The salt solution is usually prepared by admixing the monomers using a small excess of the diacid and then adding a solution of the diamine until the desired pH is obtained. Similar techniques may be used in continuous processes for preparing the salt solution for a polyamide polymerization process.
The salts required for the manufacture of the higher polyamides are less soluble in water at ambient temperature than 1,6-diaminohexane adipate salt. For example, the solubility of the salt of 1,12-diaminododecane and 1,12-dodecanedioic acid i.e. the salt for the manufacture of nylon 12/12, in water is much less than 1%, by weight, at ambient temperature. A solubility of the salt in water that is sufficient for feeding to a polymerization process is obtained only at temperatures of 120.degree.-150.degree. C. under increased pressures. However, pH cannot be monitored at these temperatures in a reliable manner; in practice, it becomes necessary to adjust the pH by small additions of one of the monomers, obtain a sample of the resultant solution and measure the pH on cooled, diluted solution. This procedure is time consuming and is not amenable to a continuous process for the manufacture of the salt.
If it is desired to use salt solutions which have relatively low water contents, to reduce the amount of water that must be removed during the subsequent polymerization process, or use molten salt obtained from molten anhydrous ingredients, then any measurements on and monitoring of the salt solution must be conducted at elevated temperatures.
The problems described above that are encountered in the production of higher polyamides or in the use of anhydrous salts are primarily those of measurement of pH at elevated temperatures. It is an illustration of the broader problem of conducting measurements of concentrations of ions at elevated temperature.
The electrodes used in the measurement of ion concentrations are often based on a conductive glass membrane. Such electrodes are used in conjunction with a reference electrode connected with the medium of the ion being measured via a salt bridge. This system is reliable at ambient or slightly elevated temperatures. However, at higher temperatures, the glass membrane tends to degrade, with the rate of this degradation depending on the nature of the medium. Thus, for example, degradation tends to be more rapid in an alkaline solution. Glass electrodes cannot be used at temperatures higher than about 130.degree. C. in mild environments and about 100.degree. C. in alkaline or other corrosive solutions.
Some electrically conductive ceramic materials can be substituted for the glass membrane in ion selective electrodes, but ceramic materials only conduct electricity at temperatures above about 180.degree. C. For example, pH sensors using stabilized zirconia ceramic membranes have been described by S. Hettiarachchi et al., J. Electrochem. Soc., 1985, 132(8), 1866.
Electrodes that do not have membranes may also be used for measuring ion concentrations in solutions at elevated temperatures. Palladium hydride was used for pH measurements by D. D. Macdonald et al., J. Electrochem. Soc. 1980, 127 (8), 1745. Similarly, use of palladium hydride electrodes was described by J. V. Dobson in U.S. Pat. No. 4,242,189, issued 1980 December 30; the state of the electrode was monitored by measuring its resistance since the resistance of palladium metal is much lower than that of palladium hydride.
The design of a high temperature reference electrode and of the salt bridge are equally important. The reference electrode may be either internal, i.e. located close to the measurement electrode at the system temperature, or external i.e. operated at ambient temperature and connected with the system by a salt bridge. Although a design described by D. D. Macdonald (J. Electrochem, Soc., 1979 (6), 908) may be suitable for measurements of short duration in relatively clean environments, performance would be expected to degrade with time, particularly in streams containing large concentrations of polymerizable or unstable components, and not be reliable for monitoring a process over an extended period of time.
A further complication of high temperature measurements of ion concentration is that of interpretation of the data obtained. The ion concentration is inferred from the measured potential of the measuring electrode. However, in equilibrium systems, such as in the measurement of pH, the measured potential will not provide useful information on the status of the system, unless the temperature dependence of the equilibrium constants involved is known. Thus, for example, the potential of a pH electrode in a "neutral" concentrated solution of a polyamide salt solution is quite dependent on the solution temperature, making it difficult to interpret and theoretically predict the amount of adjustment needed to obtain a predetermined composition.